This paper extends the application of current-mode, shared-bus converters to independently-sourced power system architectures consisting of multiple power- processing channels, each of which is configured as series-input, parallel-output (SIPO). By employing a SIPO interconnect method, current-mode commercial-off-the- shelf (COTS) DC-DC converters can transform higher input voltages into low output voltages, provide flexible options for power system expansion, and preserve system efficiencies equal to those obtained from standalone converters. Robust system stability and uniform input voltage distribution among series-connected converters is realized through input voltage distribution control. Active current-sharing control ensures nearly uniform distribution of the channel-output currents between the independently sourced SIPO power channels. Through computer simulation and experimental prototypes the uniform voltage distribution power converter architecture with current-sharing is validated and successfully applied.

The NASA's evolutionary xenon thruster (NEXT) ion propulsion system has been in advanced technology development under the NASA in-space propulsion technology project. The highest fidelity hardware planned has now been completed by the government/industry team, including: a flight prototype model (PM) thruster, an engineering model (EM) power processing unit, EM propellant management assemblies, a breadboard gimbal, and control unit simulators. Subsystem and system level technology validation testing is in progress. To achieve the objective Technology Readiness Level 6, environmental testing is being conducted to qualification levels in ground facilities simulating the space environment. Additional tests have been conducted to characterize the performance range and life capability of the NEXT thruster. This paper presents the status and results of technology validation testing accomplished to date, the validated subsystem and system capabilities, and the plans for completion of this phase of NEXT development. The next round of competed planetary science mission announcements of opportunity, and directed mission decisions, are anticipated to occur in 2008 and 2009. Progress to date, and the success of on-going technology validation, indicate that the NEXT ion propulsion system will be a primary candidate for mission consideration in these upcoming opportunities.

This paper addresses some issues concerning the application of microwave power beaming in a fractionated spacecraft environment. Some scenarios have been proposed and considered. In one scenario for example, power is distributed in free space from one "mother" spacecraft/satellite to multiple mini spacecraft/satellites in a dynamic environment, where the range and direction of transmission vary in time, and must be accounted for in the optimal energy transmission/distribution. This paper looks at various issues involved, including the type of transmitting antennas (reflectors/phased arrays), the mechanisms of redirecting and/or refocusing the single or multiple beams in the near to mid-field regions of the transmitting antenna. Receiving/rectifying antennas ("rectennas") and other issues of interest and relevance are identified and addressed. The feasibility range and areas of required improvement in terms of antenna elements and other components needed for efficient and robust performance are identified.

In this work, we are proposing a novel approach targeted to handling signal processing and data transmission tasks related to a multi-standard terminal encompassing WiMAX and UMTS standards. We shall present an innovative software-defined-radio (SDR)-based framework which will be able to arrange the digital processing tasks concerning the baseband level of the multi-standard transceiver architecture. In particular, an object-oriented model will be proposed for the baseband level implementation. The general structure consists of a number of modules for each specific and/or common component of UMTS and WiMAX signal processing chain. We start to design the SDR architecture model as a software prototype. The framework model was built as C++ class library in order to evaluate how SDR-based implementation works and to assess its quantitative performances. This model allows to consider the process of the packet interaction between transmitter and receiver and, moreover, the inter-packet interaction inside each module of the terminal. After all, the proposed software model will be integrated into the hardware platform and the final tests will be performed.

This paper presents the mechanical verification of a simplified model of a rapid byzantine-fault-tolerant self-stabilizing protocol for distributed clock synchronization systems. This protocol does not rely on any assumptions about the initial state of the system except for the presence of sufficient good nodes, thus making the weakest possible assumptions and producing the strongest results. This protocol tolerates bursts of transient failures, and deterministically converges within a time bound that is a linear function of the self-stabilization period. A simplified model of the protocol is verified using the symbolic model verifier (SMV). The system under study consists of 4 nodes, where at most one of the nodes is assumed to be Byzantine faulty. The model checking effort is focused on verifying correctness of the simplified model of the protocol in the presence of a permanent byzantine fault as well as confirmation of claims of determinism and linear convergence with respect to the self-stabilization period. Although model checking results of the simplified model of the protocol confirm the theoretical predictions, these results do not necessarily confirm that the protocol solves the general case of this problem. Modeling challenges of the protocol and the system are addressed. A number of abstractions are utilized in order to reduce the state space.

This contribution aims at unifying two recent trends in applied particle filtering (PF). The first trend is the major impact in simultaneous localization and mapping (SLAM) applications, utilizing the FastSLAM algorithm. The second one is the implications of the marginalized particle filter (MPF) or the Rao-Blackwellized particle filter (RBPF) in positioning and tracking applications. Using the standard FastSLAM algorithm, only low-dimensional vehicle models are computationally feasible. In this work, an algorithm is introduced which merges FastSLAM and MPF, and the result is an algorithm for SLAM applications, where state vectors of higher dimensions can be used. Results using experimental data from a UAV (helicopter) are presented. The algorithm fuses measurements from on-board inertial sensors (accelerometer and gyro) and vision in order to solve the SLAM problem, i.e., enable navigation over a long period of time.

A two terminal laser communication test bed has been developed at The Aerospace Corporation. This paper presents the design and preliminary results of a reprogrammable detector within the Test Bed for use in pointing, acquisition, and tracking between a satellite-to- satellite laser communication link. The detector may be commanded by an emulated spacecraft Command & Data Handling subsystem to switch between full-array scanning and "small sized" N x M pixel Field of View (FOV) for high-rate laser tracking. The approach follows a parallel path to implement the signal processing algorithm on two different hardware resources: a Field Programmable Gate Array (FPGA) and a Digital Signal Processor (DSP). The focus of this effort is to present a methodology for testing and evaluating various techniques for advanced focal plane array (FPA) hardware, as well as sensor FPA control, image processing and laser beam X & Y position algorithms.

A system in which both processed data uplink (PDU) and sensor data (SD) downlink coexist within the 8 GHz band is one of several alternatives being considered for the GOES-R frequency plan. The 8 GHz studies performed so far indicate that having both PDU and SD downlink within the 8175-8400 MHz band is achievable. In such a design, a number of issues must be addressed related to interference to and from other systems in the neighborhood of the GOES-R frequency bands. These issues are self-interference from SD downlink transmitter to PDU receivers on the satellite, hardware complexity such as filter design, and payload configuration, for example, use of a single antenna for both SD downlink and PDU receive on the satellite or two separate antennas. Among the interference issues, the important ones are PDU and SD downlink self- interference, SD downlink interference to the deep space network (DSN) at Goldstone, PDU to DISA, and DISA shipboard terminal interference to PDU. In this paper we give a brief overview of the GOES-R system, followed by a detailed discussion of the design issues. A reference design that satisfies the link performance, interference limits, and implementable hardware constraints is given in this paper.

Next generation satellite payloads must optimize the use of available spectrum to deliver the highest possible data rates while maintaining a required quality of service (QoS). The satellite channel is highly variable due to changes in the environment such as cloud cover, rain, etc. Mobile ground terminals introduce another degree of channel volatility. The dynamic bandwidth resource allocation (DBRA) technique addresses the channel dynamics by continuously monitoring key link parameters and adaptively optimizing the physical layer parameters. These modifiable parameters include: symbol rate, modulation, error correction code rate, spreading factor, transmit power, etc. A flexible radio that supports all of these parameters is coupled to a dynamic channel emulator to evaluate DBRA system performance. A complete DBRA system must also address the networking layer and even the application layer. As the satellite channel conditions change, all layers must respond appropriately to maintain QoS and avoid exceeding available data rates. Poor coordination can result in dropped calls or worse. This paper describes a DBRA testbed with multiple audio and video codecs that are selected without renegotiation. Experimental results and 'lessons learned' are presented. A brief background on modern video and audio codecs is also provided.

The Space Interferometry Mission Planet Quest Light (SIM PQL) is a new concept for a space borne astrometric instrument. It will be located in a solar Earth-trailing orbit. SIM PQL utilized technology developed for the space interferometry mission planet quest (SIM PQ). The instrument consists of two Michelson stellar interferometers and a telescope. The first interferometer chops between the target star and a set of reference stars. The second interferometer monitors the attitude of the instrument in the direction of the target star. The telescope monitors the attitude of the instrument in the other two directions. SIM PQL will be capable of one micro-arc-second narrow angle astrometry, over a two-degree field of regard for magnitude 6 and brighter target stars. During the 5-year mission, SIM PQL would search 50 nearby stars for planets of mass down to one Earth mass, in the Habitable Zone, which have orbit periods of less than 3 years. SIM PQL will also perform global astrometry on a variety of astrophysics objects, reaching 6 micro-arc-seconds absolute position and parallax measurements. As a pointed instrument, SIM PQ will maintain its astrometric accuracy on fainter objects. This paper will describe the instrument, how it will do its astrometric measurements and the expected performance based on the current technology.